A prior invention describes intumescent, flame retardant compositions comprising a thermoplastic polyurethane resin; a nitrogen-containing phosphate, selected from amine phosphates, ammonium phosphates, and ammonium polyphosphates; and a pentate salt selected from melammonium pentate and pentate salt of ammelide. Such compositions contain 10-60% by weight of the flame retardants and 90-40% by weight polyurethane resin. The ratio of the flame retardants is about 10-99% by weight of the nitrogen-containing phosphate to about 90-1% of the pentate salt. An aryl phosphate can also be used together with the two flame retardants.
The polyurethane resin referred to above is free of halogens and for certain applications should have an oxygen index (OI) of 29 or greater and UL94 rating of V0, along with other requirements. Although these requirements were met and were exceeded by the compositions noted above, which were prepared on a relatively small scale, scale-up of the preparation of the compositions surprisingly and unexpectedly yielded very disappointing results. Scaled-up compositions have shown a substantial drop in the oxygen index as well as in UL94 rating. It appears that regardless of the level of flame retardants used, continuous compounding under pressure in the scale-up experiments caused reduction in oxygen index to about 27 and increased flame time in the UL94 Vertical Burn Test. For instance, polyurethane compositions formulated on a small scale with increasing amount of the flame retardants, had oxygen rating of 32, 34 and 36 whereas scaled-up compositions containing the same amount of the same flame retardants, had oxygen index of only 27. Likewise, with the UL 94 ratings, whereas the small scale compositions gave rating of V0, the scaled-up compositions gave evaluations of V2 or the samples failed by other criteria, such as burning to the clamp. The following table summarizes the results for the small scale and scaled-up compositions containing three levels of flame retardants consisting of a pentate salt and a nitrogen-containing polyphosphate. The table also dramatizes the loss of flame retardancy of the scaled-up compositions:
______________________________________ Flame Retardancy Tests Compositions OI UL 94 ______________________________________ small scale 32 34 36 V0 V0 V0 scaled-up 27 27 27 None None V2 ______________________________________
The term "None" in the above table indicates that the sample failed the test by another criterion. In this case, the samples burned to the clamp.
A great deal of thought has been given to the poor performance of the scaled-up compositions. The reason for the sub-par performance appears to reside in the amount of processing a composition undergoes during its preparation. Whereas the small scale batch compositions experienced a lesser amount of processing, the scaled-up compositions were prepared in continuous equipment which imparted a significantly higher rate of shear and significantly higher pressures which may have led to some form of a prereaction between the ingredients of the compositions. This appears to be a plausible explanation of what may have taken place during preparation of the scaled-up compositions since the invention described and claimed herein appears to have improved the flame retarding performance in terms of oxygen index and the UL 94 test of the scaled-up compositions.
Although there is no rigorous cause and effect understanding of what happened when the small scale ba tch preparation of the compositions was scaled-up to continuous production on a larger scale, the conclusions drawn herein are based on observed occurrences. For instance, it was observed that there was odor evolution from the scaled-up compositions. This means that a reaction was taking place which was not the case with the small scale, batch production. The degree of damage to the flame retarding properties of the flame retardants appears to be related primarily to the rate of shear during processing, which is a measure of particle--particle collisions between the two flame retardants. Temperature and pressure may also be contributing factors.
The invention referred to above is described and claimed in U.S. Pat. No. 4,542,170 issued Sept. 17, 1985 and entitled "Intumescent Flame Retarded Polyurethane Compositions" of the inventors D. R. Hall and R. L. Jackson. This patent is incorporated by reference as if fully set forth herein.